Thermo-mechanical transducer



Feb. 27, 1968 A. H. RICH 3,371,309

THERMO-MECHANICAL TRANSDUCER Filed June 10, 1965 2 sheetssheet 1INVENTOR ALAN H. RICH BY Q48 ATTORNEY Feb. 27, 1968 A. H. RICHTHERMO-MECHANICAL TRANSDUCER 2 Sheets-Sheet 2 Filed June 10, 1965 0 0 mu0 a w m cry wmpzmumzmh zortmzeih INVENT OR 1 CHROMIUM CONCENTRATIONATOMS (X) ALA/V H RICH ATTORNEY United States Patent 3,371,39 PatentedFeb. 27, 1968 3,371,309 THERMO-MEiIHANiCAL TRANSDUCER Alan H. Rich,Washington, D.C., assignor to the United States of America asrepresented by the Secretary of the Navy Filed June 10, 1965, Ser. No.463,058 3 Claims. (Cl. 340-8) ABSTRACT OF THE DISCLGSURE Athermo-mechanical transducer utilizing chromium modified manganeseantimonide as the driver element wherein the chromium modified manganeseantimonide is characterized by the property of expanding in a sharp,step-like manner as the temperature of the material passes upwardthrough an interval of a very few degrees about a given temperature andof contracting in a similar manner as the temperature passes downwardthrough this interval so as to create mechanical motion which can beused to create compressional waves.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governm ntal purposeswithout the payment of any royalties thereon or therefor.

The present invention relates to a transducer and more particularly to alow frequency acoustic transducer which is capable of producing largeamplitude compressional waves.

In recent years, low frequency acoustic transducers have becomeincreasingly important for both military and civilian purposes. Mucheffort has been expended on developing magnetostrictive andpiezoelectric drivers suitable for such transducers with only limitedsuccess.

Magnetostrictive drivers for low frequency transducers are expensive andineflicient. When it is desired to obtain a large amplitude, lowfrequency, compressional wave output, the magnetostrictive drivingsource and, hence the transducer, must be physically large. Furthermore,large currents are necessary to create the requisite magnetic field.

Piezoelectric drivers for low frequency transducers are expensive,diificult to fabricate and also inetficient. Moreover, thesedisadvantages are all aggravated when the transducer is designed toproduce not only a low freuency but also a large amplitude compressionalwave.

The present invention overcomes these disadvantages by approaching theproblem of drivers for low frequency transducers in a novel manner. Thedriver of the instant invention does not employ conventionalmagnetostrictive or piezoelectric principles but, rather, converts heatinto mechanical motion which motion can be used to produce compressionalwaves. Chromium modified manganese antimonide having the general formulaMn Cr Sb, where x equals the concentration of chromium in atoms and0.025 0.20, can be used as the driver. Crystals composed of thiscompound and similar compounds have theproperty of expanding in a sharp,step-like manner as the temperature of the crystal passes upward throughan interval of a very few degrees about a given temperature and ofcontracting in a similar manner as the temperature of the crystal passesdownward through this interval. In chromium modified manganeseantimonide, this given temperature is the temperature at which thechromium modified manganese antirnonide goes through a transition from aweakly magnetic to a ferrimagnetic state. This property is used tocreate mechanical motion which can be used in an acoustic transducer tocreate compressional waves. Inasmuch as the amount of me chanical motioncreated by the expansion and contraction of the crystal is dependent onthe size of the crystal, a crystal can be chosen to producecompressional waves of any amplitude desired.

An object of the present invention is the provision of a simple,inexpensive, and efficient low frequency transducer.

Another object is to provide a compact low frequency transducer which iscapable of producing large amplitude compressional waves and which iseasy to fabricate.

A further object of the invention is the provision of a transducer whichconverts thermal energy into mechanical energy.

Still another object is to provide a compact underwater acoustictransducer which is simple to construct and capable of producing largeamplitude compressional waves and is driven by a crystal which convertsenergy in the form of heat into mechanical motion.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings in which like referencenumerals designate like parts throughout the figures thereof andwherein:

FIG. 1 shows a longitudinal cross-section of a cylindrical transducerincorporating the invention;

FIG. 2 illustrates a transverse section of the transducer taken alongthe line 22 of FIG. 1; and

FIG. 3 is a graph showing how the transition temperature varies with theamount of chromium present in the compound.

Referring now to FIG. 1, cylindrical crystal 11 forms the driver forcylindrical piston transducer 12. Crystal 11 is composed of a compoundsuch as chromium modified manganese antimonide having the generalformula IvIi1 Cr Sb, where x equals the atoms of chromium and 0.0250.20. The ends of crystals 11 are butted against pistons 13 which may beformed from stainless steel. Preferably, pistons 13 have shank portions14 which have relatively small diameters and have bores 28 formedtherein to receive the ends of crystal 11 and face portions 15 whichhave relatively large diameters and are as thin as possible giving dueconsideration to the hydrostatic pressure that will be bearing againstthe transducer during operation. This construction has the advantages ofholding crystal 11 firmly in place, having a large effectivetransmitting area 15, and having as little frictional contact betweenpistons 13 and sleeve 16 as possible.

Sleeve 16 which may also be formed from stainless steel, makes slideablecontact with the peripheries of piston faces 15 and thereby preventsmotion of the pistons transverse to the longitudinal axis of thecylinder. The surface of sleeve 16 which contacts faces 15 may be coatedwith a low coefficient of friction material such as Teflon to preventseizure between the sleeve and the piston face. Pistons 13 are retainedwithin sleeve 16 by retaining members 17 which may be snap rings.Flexible, fluid-impermeable sealing member 18, which may be rubber,encloses piston faces 15 and sleeve 15 thereby preventing flooding ofthe transducer.

Heating coil 19 is wound about the longitudinal axis of crystal 11,preferably throughout the full length of the crystal, on an electricalinsulating, thermal conducting member 20 which may be ceramic. Theamount of heat supplied by heating coii 19 is controlled by the amountof current flowing therethrough. This current may comprise an A.C.component supplied by power oscillator 21 superimposed on a DC.component supplied by DC. source 22.

When crystal 11 is chromium modified manganese antimonide, there is achange in length of crystal 11 of approximately 0.20% that occurs in astep-like manner as the temperature of the crystal passes through aninterval of approximately 3 K. about the transition temperature. It isthis step-like change in length that is used to drive piston 15.Clearly, the magnitude of the compressional waves that will be producedby the transducer is dependent on the length of crystal 11. Theapproximate 0.20% change in length of crystal 11 that occurs over thetemperature interval means that the length of crystal 11 can besignificantly less than the length of a magnetostrictive driver used toproduce compressional waves of the same amplitude.

In operation, D.C. source 22 is adjusted to supply suflicient current toheating coil 19 to cause it to generate suflicient thermal energy tohold the temperature of crystal 11 just below the lowest temperature ofthe aforementioned temperature interval. Oscillator 21, which is coupledto heating coil 19 through a transformer 23 to prevent interactionbetween oscillator 21 and source 22, supplies an AC. current componentwhich causes heating coil 19 to generate sufiicient additional thermalenergy to cause the temperature of crystal 11 to pass through thistemperature interval. Pistons 13 and sleeve 16 act as a heat sink forcrystal 11 and thermally couple the crystal to the environment in whichthe transducer is used. Thus, if the temperature of the environment isbelow the lowest temperature of the temperature interval, thetemperature of crystal 11 will be returned to a temperature below thelowest temperature of this interval after being increased through theinterval by the heat transfer between the crystal and the environment.

The upper frequency of transducer 12 is limited by the rapidity of thisrecovery. When the transition temperature of crystal 11 is as high aspossible, approximately 400 K., transducer 12 can operate at a maximumfrequency of approximately c.p.s. if no cooling provisions are madeother than the heat transfer between the crystal and the environment.This means that the oscillator 21 can have a frequency up to 10 c.p.s.If it is desired to operate at a lower frequency, the transitiontemperature can be controlled by varying the amount of chromium asappears from FIG. 3.

Of course, if separate cooling provisions were made, such as if thecrystal were exposed to a refrigerant during the recovery period,transducer 12 could operate at a higher frequency.

It should be understood that the current source for heating coil 19could be contained within transducer 12. In such an embodiment, the DC.component could be provided by a battery and the AC. component could besupplied by a second battery coupled to the heating coil through achopper that makes and breaks contact at the desired frequency ofoperation.

Battery 24 furnishes current for a monitoring circuit that includesthermistor 25 and current sensitive meter 26. Thermis-tor 25 is enclosedin a thermally nonconductive envelope 27 on all sides except for theside disposed toward crystal 11 and is thermally coupled to the crystalso that its resistance will vary solely as a function of the variationsin the temperature of the crystal. Current sensitive meter 26 iscalibrated in terms of temperature so that it will provide a directreading of the temperature of crystal 11. Using this circuit, one maymonitor the temperature of crystal 11 and adjust the magnitude of thecurrent supplied by DC. source 22 so that the AC. signal supplied byoscillator 21 will always cause the temperature of the crystal to varythrough the aforementioned temperature interval. Of course, themonitoring and adjusting could be done automatically.

In practice, the leads from heating coil 18 and thermistor 23 passthrough sleeve 16 and seal 18 in a fluid tight fitting (not shown).

FIG. 2 shows a transverse section of transducer 12 4 taken along line 22in FIG. 1. It will be observed that in a cylindrical transducer, such asillustrated, the crystal 11, piston 13, electrical insulating, thermalconducting member 20, heating coil 19, sleeve 16, and seal 18 are allconcentric.

It should be noted that compounds such as chromium modified manganeseantimonide could be used as the driver in other than cylindricaltransducers. The only limitation that exists on the type of transducerthey could be the driver for is that it is necessary that a source ofheat be present which can vary the temperature of the crystal throughthe interval about its transition temperature and that there be means todissipate heat away from the crystal.

As can be seen from FIG. 3, the transition temperature for a crystalcomposed of chromium modified manganese antimonide having the generalformula Where x equals the atoms of chromium, which is a typicalcompound for the driver according to this invention, varies from alittle above K. to a little below 400 K. as X varies from 0.025 to 0.20atoms.

One possible use of transducers with drivers according to the presentinvention is as underwater acoustic transducers, which are typicallyused in an environment having an ambient temperature of approximately280 K. In the case where the driver is composed of chromium modifiedmanganese antimonide and there is to be no separate provision made forcooling the crystal, the chromium concentration in atoms will have to begreater than approximately 0.10. In this way the environment will beable to provide the cooling necessary to bring the temperature of thecrystal below the lowest temperature of the approximate 3 K. intervalabout the transition temperature after it is raised above the highesttemperature of the interval to produce the desired step-like expansion.As the atoms of chromium are increased up to 0.20, it is possible toobtain a larger difference between the transition temperature of thecompound and the temperature of the environment. The larger thisdifference, the higher the frequency at which the transducer beingdriven by a crystal composed of the compound can operate. As mentionedbefore, the maximum frequency of operation is approximately 10 c.p.s.unless separate cooling provisions are made.

It should now be clear that the present invention provides a simple,compact, low frequency transducer capable of producing large amplitudecompressional waves.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood, that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is: j

1. A thermo-mechanical sonic transducer comprising:

a driver crystal composed of chromium modified manganese antimonidehaving the general formula Mn Cr Sb Where x equals the concentration ofchromium in atoms and 0.0255X5020 and wherein said chromium modifiedmanganese antimonide has the property of expanding in .a step-likemanner as the temperature thereof is increased from a first temperatureto a second temperature and of contracting in a step-like manner whenthe temperature thereof is decreased from said second temperature tosaid first temperature;

an electrical heater thermally coupled to said driver crystal;

a direct current heating signal source electrically coupled to saidheater, and adapted to pass a first 'sig nal through said heater of sucha magnitude that the thermal energy generated thereby heats the crystalto a temperature just below said first temperature;

means electrically coupled to said heater for passing a second heatingsignal therethrough, the magnitude of said second signal periodicallyvarying at sonic frequencies from a first value to a second valuewherein said first value is below the signal magnitude necessary tocreate sufficient additional thermal energy to increase the temperatureof said crysml to said second temperature, and said second value is atleast equal to the signal magnitude necessary to create suflicientadditional thermal energy to increase the temperature of said crystal tosaid second temperature;

means thermally coupled to said crystal for monitoring the temperaturethereof;

means for adjusting the magnitude of said first signal in response tothe temperature of said crystal that is sensed by said monitoring meansso that the temperature of said crystal always varies from said firsttemperature to said second temperature when the magnitude of said secondsignal varies from said first value to said second value; and

cooling means thermally coupled to said crystal for periodicallydecreasing the temperature thereof from said second temperature to saidfirst temperature sequentially with respect to the increasing of thetemperature of said crystal from said first temperature to said secondtemperature so as to enable the heating of said crystal to said secondtemperature and the cooling of said crystal to said first temperature atsonic frequencies.

2. The transducer of claim 1 for use in an environ- References CitedUNITED STATES PATENTS 2,962,695 11/1960 Harris 34011 X 3,126,347 3/1964Swoboda 252-625 3,140,942 7/1964 Walter 75122 3,198,969 8/1965 Kolm etal 3104 3,238,396 3/1966 Schubring et a1. 310-4 OTHER REFERENCES Swobodaet al.: Evidence for an Antiferrornagnetic Ferrimagnetic Transition inCr-Modified Mn Sb, Physical Reviews, May 15, 1960, pp. 509-511.

Cloud et 211.: Exchange Inversion in Mn Cr Sb, Journal of AppliedPhysics, Supplement, March 1961, pp. S56S.

RODNEY D. BENNETT, Primary Examiner.

RICHARD A. FARLEY, Examiner.

B. L. RIBANDO, Assistant Examiner.

